1. Field of the Invention
The present invention relates to an overlay alloy used for a plain bearing. More particularly, the present invention relates to an overlay alloy which is composed mainly of Pb, exhibits excellent corrosion-resistance and fatigue-resistance, and, in particular, slides on a crank shaft of an internal combustion engine.
2. Description of Related Arts
The plain bearing of an internal combustion engine is embodied as a combination of a cylindrical bush, a half cylindrical bearing and a thrust bearing. A plain bearing consists of a backing strip which is usually made of steel, a plain bearing alloy which is bonded to the backing strip and which is usually referred to as a lining, and an overlay bonded to the lining.
The overlay alloy is a soft alloy mainly composed of lead or lead and tin. Improvements in its properties have been made by devicing various additives.
The plain bearing shown in Japanese Examined Patent Publication No. 39-22498, which corresponds to G.B. Patent No. 989,946, is one in which an overlay alloy which contains from 2 to 10% by weight of In, and further at least one optional additive element, such as from 0.1 to 3% by weight of Cu, from 0.001 to 0.25% by weight of Te, not more that 0.5% by weight of Ag, and not more than 0.5% by weight of Sb, the balance being Pb, is bonded on the lining which consists of a Cu-Pb alloy. In, Cu and Te allegedly improve the corrosion-resistance of the cast alloy according to the description of the above publication.
An overlay alloy shown in Japanese Unexamined Patent Publication No. 61-266,544 contains from 1 to 10% by weight of In, and from 0.1 to 6% by weight of Cu, as well as not more than 8% by weight of Sn as the optional additive element, the balance being Pb. Cu is allegedly effective for refining the surface layer of the alloy and retarding the diffusion of In into the underlying layer, thereby improving the corrosion-resistance and the fatigue-resistance according to the description of the publication. In allegedly improves corrosion-resistance, particularly when Sn and In are both present.
The present assignee proposed in Japanese Unexamined Patent Publication No. 59-205,442 an overlay alloy which contains from more than 10% to 15% by weight of In and/or Tl, from 2 to 10% by weight of Sn, and from 0.05 to 5% by weight of one or more of Cu, Sb, Mn, Ni, Ca, Bi, and Ba. Allegedly, In and Tl mainly improve the corrosion-resistance, and Cu and the like form intermetallic compounds and also mainly improve the corrosion-resistance.
Since internal combustion engines in recent years are being operated at higher temperatures, under heavier-loads, and over longer periods of time, the properties required of an overlay become more and more severe.
Sn is contained as an essential element of the overlay disclosed in Japanese Unexamined Patent Publication No. 59-205,442, and is also contained as an optional element of the overlay disclosed in Japanese Unexamined Patent Publication No. 61-266,544. Sn can enhance the corrosion-resistance and fatigue-resistance in an ordinary Pb-based overlay. The thus enhanced corrosion-resistance and fatigue resistance deteriorate during service of a plain bearing because Sn diffuses from the overlay to the lining. A Ni barrier layer, which is interposed between the lining and the overlay of an ordinary bearing so as to prevent the Sn diffusion, becomes ineffective at the high tempertures at which internal combustion engines are recently operated. Notwithstanding the presence of the Ni plating layer, Sn diffuses from the overlay to the lining, with the result that Sn is deficient and, further, crystal defects, such as vacancies, generate in the overlay. As a result, the corrosion-resistance and fatigue-resistance deteriorate. Note that the Sn and Ni actively react with one another at a high temperature, and, therefore, the Sn plating layer is a drawback in enhancing the properties of an overlay used at a high temperature.
Cu, which is added in the overlay alloy of Japanese Unexamined Patent Publication No. 59-205,442, also reacts with Ni of the diffusion-barrier layer at a high temperature, and hence diffuses into the lining. As a result, Cu becomes deficient in the overlay, thus impairing fatigue resistance. In addition, In and Sn, which are added together in the overlay of said publication, cause drastic reduction of the melting point or solidus so that the diffusion of Sn and Cu is very liable to occur.
Since Cu and In are essential in the overlay shown in Japanese Unexamined Patent Publication No. 61-266,544, the diffusion of Sn and Cu is very liable to occur as described above.
The overlay shown in Japanese Examined Patent Publication No. 39-22498 is the subject matter of an application based on the Convention Priority on 1960. At that time, conditions under which the internal combustion engine was used were considerably less severe than at present. The Ni barrier is not mentioned in said publication. It seems rather that the In diffusion of the overlay into the lining and the positive Cu diffusion of the lining into the overlay are merely mentioned.